1. Field of the Invention
The present invention relates generally to propulsors for surface ships and, more particularly, to a tractor podded propulsor unit for surface ships having contrarotating propellers mounted at the forward end of a streamlined pod that is aligned with the local incoming flow.
2. Brief Description of Related Art
A critical operating problem associated with surface ships, particularly high speed vehicles, is the existence of propeller blade cavitation. Operated below the free surface, a propeller will develop vortex cavitation and surface cavitation on the blade above a certain critical speed. Cavitation inception occurs when the local pressure falls to or below the vapor pressure of the surrounding fluid. Above the critical cavitation inception speed, serious fundamental flow changes occur that lead to undesirable variations in hydrodynamic and acoustic characteristics and possible damage to blade structure. Specifically, rudder cavitation induces unsteady hydrodynamic forces, vibration, and erosion resulting in noise, thrust breakdown, and blade erosion, all of which are detrimental to ship performance.
Conventional, single rotation propulsors mounted on inclined, strut supported shafts are the typical propulsion systems found on present surface ships. By mounting propellers on inclined shafts, the propeller experiences inflow at a nominal flow angle generally equal to the difference between the inclined shaft angle and the aft buttock lines. Moreover, because the shaft and strut are forward of the propeller, they induce nonuniform inflow into the propeller. This inclined, nonuniform flow results in a blade angle of attack variations that contribute to early blade cavitation.
Consequently, there is a need to provide a propulsor that reduces the detrimental effects of cavitation. More particularly, it would be desirable to provide a propulsor that reduces nonuniformities in the inflow. A more uniform inflow would result in the propeller blade section experiencing a nearly constant angle of attack which would improve cavitation performance by increasing cavitation inception speed.